Imaging lens

ABSTRACT

An imaging lens includes a first lens having positive refractive power; a second lens having negative refractive power; a third lens having positive refractive power; and a fourth lens having positive refractive power, arranged from an object side to an image plane side. In the first lens, a curvature radius on an object-side surface is positive and a curvature radius of an image-side surface is negative. In the second lens, curvature radii of an object-side surface and an image-side surface are both positive. In the third lens, curvature radii of an object-side surface and an image-side surface thereof are both negative. When the whole lens system has a focal length f and a distance from the object-side surface of the first lens to an image-side surface of the fourth lens is L14, the imaging lens satisfies the following expression:
 
0.5&lt; L 14/ f &lt;0.8

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of the prior PCT applicationPCT/JP2010/068442, filed on Oct. 20, 2010, pending, which claimspriority from a Japanese patent application No. 2009-243049, filed onOct. 22, 2009, the entire content of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image onan imaging element such as a CCD sensor and a CMOS sensor. Inparticular, the present invention relates to an imaging lens suitablefor mounting in a relatively small camera such as a cellular phone, adigital still camera, a portable information terminal, a securitycamera, an onboard camera, and a network camera.

An imaging lens for mounting in the above-described small-sized camerais required to have a small size, as well as sufficient opticalperformances that can be compatible with recent imaging elements withhigh resolution. Conventionally, when an imaging lens did not have highresolution, it was possible to attain both sufficient opticalperformances suitable for resolution of the imaging element andminiaturization even with an imaging lens having a two-lens orthree-lens configuration. However, as resolution of an imaging lensbecomes higher, required optical performances become higher, so that itbecomes difficult to attain both sufficient optical performances withsatisfactorily corrected aberration and miniaturization.

For this reason, there have been studies in adding another lens, i.e. animaging lens with a four-lens configuration. For example, an imaginglens described in Patent Reference includes in this order from an objectside, a first lens that has a convex shape on the object side and ispositive; a second lens that has a shape of a negative meniscus lensdirecting a concave surface thereof to the object side; a third lensthat has a shape of a positive meniscus lens directing a convex surfacethereof to the object side; and a fourth lens that has a shape of apositive meniscus lens directing a convex surface thereof to the objectside. According to this configuration, satisfactory optical performancesare obtained while restraining increase of the total length of theimaging lens by setting preferred ranges for a ratio of a focal lengthof each of the first to the third lenses to a focal length of the lenssystem, for refractive index of the first lens, and for Abbe's number ofthe first lens, and then respectively keeping those values within theranges.

Patent Reference: Japanese Patent Publication No. 2007-122007

According to the imaging lens described in Patent Reference, it ispossible to attain relatively satisfactory aberrations. In each year,however, there have been advancements in miniaturization andperformances of devices themselves for mounting the above-describedsmall-sized cameras, so that the size required for such imaging lens hasbeen even smaller than before. In case of the lens configurationdescribed in Patent Reference, it is difficult to attain bothminiaturization and satisfactory aberration correction, so as to meetthe above-described requirements.

In view of the problems of the conventional techniques described above,an object of the present invention is to provide an imaging lens thatcan satisfactorily correct aberrations in spite of a small size thereof.

SUMMARY OF THE INVENTION

In order to attain the object described above, according to the presentinvention, an imaging lens includes a first lens having positiverefractive power; a second lens having negative refractive power; athird lens having positive refractive power; and a fourth lens havingpositive refractive power, arranged in this order from an object side toan image plane side. Furthermore, the first lens is formed in a shape sothat a curvature radius of an object-side surface thereof is positiveand a curvature radius of an image-side surface thereof is negative. Thesecond lens is formed in a shape so that a curvature radius of anobject-side surface thereof and a curvature radius of an image-sidesurface thereof are both positive. The third lens is formed in a shapeso that a curvature radius of an object-side surface thereof and acurvature radius of an image-side surface thereof are both negative. Inaddition, when a whole lens system has a focal length f, a distance onan optical axis from the object-side surface of the first lens to theimage-side surface of the fourth lens is L14, the imaging lens satisfiesthe following conditional expression (1):0.5<L14/f<0.8  (1)

Moreover, according to the present invention, an imaging lens includes afirst lens having positive refractive power; a second lens havingnegative refractive power; a third lens having negative refractivepower; and a fourth lens having positive refractive power, arranged inthis order from an object side to an image plane side. The first lens isformed in a shape so that a curvature radius of an object-side surfacethereof is positive and a curvature radius of an image-side surfacethereof is negative. The second lens is formed in a shape so that acurvature radius of an object-side surface thereof and a curvatureradius of an image-side surface thereof are both positive. The thirdlens is formed in a shape so that a curvature radius of an object-sidesurface thereof and a curvature radius of an image-side surface thereofare both negative. In addition, when a whole lens system has a focallength f, a distance on an optical axis from the object-side surface ofthe first lens to the image-side surface of the fourth lens is L14, theimaging lens satisfies the following conditional expression (1):0.5<L14/f<0.8  (1)

When the imaging lens with the above configuration satisfies theconditional expression (1), it is possible to reduce a length(thickness) of the imaging lens along the optical axis whilesatisfactorily correcting an aberration.

In the conditional expression (1), when the value exceeds the upperlimit “0.8”, the distance on the optical axis from the object-sidesurface of the first lens to the image-side surface of the fourth lensis long in relative to the focal length, and it is difficult to attainminiaturization of the imaging lens. On the other hand, if the value isless than the lower limit “0.5”, although it is advantageous forminiaturization of the imaging lens, the thickness of each lens thatcomposes the imaging lens is extremely thin, so that the fabricationproperties and productivity are significantly lowered. In addition, itis difficult to satisfactorily correct aberrations.

When a distance on the optical axis from an image-side surface of thesecond lens to an object-side surface of the third lens is d23 and adistance on the optical axis from an object-side surface of the secondlens to an image-side surface of the fourth lens is L24, the imaginglens having the above-described configuration preferably satisfies thefollowing conditional expression (2):0.3<d23/L24<0.7  (2)

When the imaging lens satisfies the conditional expression (2), it ispossible to restrain an incident angle of a light beam emitted from theimaging lens to an imaging element within a certain range and restrainfield curvature within satisfactory range, while shortening thethickness of the imaging lens. As well known in the art, for light beamsthat an imaging element can take, a maximum incident angle is set as anincident angle limit in view of an imaging element structure. A lightbeam outside the range of the maximum incident angle may result in animage with a dark periphery due to a shading phenomenon. For thisreason, it is necessary to restrain the incident angle of a light beamemitted from the imaging lens to the imaging element within the certainrange.

When the value exceeds the upper limit “0.7”, although it isadvantageous to restrain an incident angle of a light beam emitted fromthe imaging lens to an imaging element within a certain range, sinceeffective diameters of the third lens and the fourth lens increase, itis difficult to attain miniaturization of the imaging lens. In addition,since an astigmatic difference increases, it is difficult to obtain aflat image surface. On the other hand, when the value is below the lowerlimit “0.3”, although it is advantageous for miniaturization of theimaging lens, it is difficult to restrain an incident angle of a lightbeam emitted from the imaging lens to an imaging element within acertain range.

When a composite focal length of the first lens and the second lens isf12 and a composite focal length of the third lens and the fourth lensis f34, the imaging lens having the above-described imagingconfiguration preferably satisfies the following conditional expression(3):0.1<f12/f34<0.8  (3)

When the imaging lens satisfies the conditional expression (3), it ispossible to reduce the thickness of the imaging lens and restrain therespective aberrations, which includes off-axis chromatic aberration ofmagnification, within satisfactory ranges in a well-balanced manner.When the value exceeds the upper limit “0.8”, the composite focal lengthof the first lens and the second lens is long in relative to thecomposite focal length of the third lens and the fourth lens, and aposition of a principal point of the lens system moves to the imageplane side, so that it is difficult to attain miniaturization of theimaging lens. Furthermore, an off-axis chromatic aberration ofmagnification is insufficiently corrected (that of a short wavelengthincreases in a minus direction in relative to that of a referencewavelength), which makes it difficult to obtain satisfactory imagingperformance. On the other hand, when the value is below the lower limit“0.1”, the composite focal length of the first lens and the second lensis short in relative to the composite focal length of the third lens andthe fourth lens, and refractive power of the lens system concentrates onthe first lens and the second lens, so that it is difficult to restraina spherical aberration and a coma aberration within satisfactory rangesin a well-balanced manner. Moreover, the incident angle of an off-axislight beam emitted from the imaging lens to the imaging elementincreases, so that it is difficult to restrain an incident angle of alight beam emitted from the imaging lens to the imaging element within acertain range.

The imaging lens having the above-described configuration furtherpreferably satisfies the following conditional expression (3A):0.2<f12/f34<0.6  (3A)

When the first lens has Abbe's number νd1 and the second lens has Abbe'snumber νd2, the imaging lens having the above-described configurationpreferably satisfies the following conditional expressions (4) and (5):νd1>50  (4)νd2<30  (5)

When the imaging lens satisfies the conditional expressions (4) and (5),it is possible to satisfactorily correct chromatic aberrations. When theAbbe's number of the first lens or the Abbe's number of the second lensis outside the range of the conditional expression (4) or (5), the axialchromatic aberration is insufficiently corrected, and it is difficult toobtain satisfactory imaging performance.

When the third lens has Abbe's number νd3 and the fourth lens has Abbe'snumber νd4, the imaging lens having the above-described configurationpreferably satisfies the following conditional expressions (6) and (7):|νd1−νd4|<10  (6)|νd2−νd31<10  (7)

When the imaging lens satisfies the conditional expressions (6) and (7),it is possible to further satisfactorily correct the axial chromaticaberration and off-axis chromatic aberration of magnification.

Furthermore, in the imaging lens having the above-describedconfiguration, when a material of the first lens and a material of thefourth lens are same and a material of the second lens and a material ofthe third lens are same, the number of types of materials to compose theimaging lens is only two, so that it is possible to reduce themanufacturing cost of the imaging lens.

According to the imaging lens of the invention, it is possible to attainboth miniaturization of the imaging lens and satisfactory aberrationcorrection, and it is possible to provide a small-sized imaging lenswith satisfactorily corrected aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 1 according to a first embodimentof the invention;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens of Numerical Data Example 1;

FIG. 3 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of Numerical DataExample 1;

FIG. 4 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 2 according to the firstembodiment of the invention;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens of Numerical Data Example 2;

FIG. 6 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of Numerical DataExample 2;

FIG. 7 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 3 according to the firstembodiment of the invention;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens of Numerical Data Example 3;

FIG. 9 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of Numerical DataExample 3;

FIG. 10 is a schematic sectional view showing a configuration of animaging lens in Numerical Data Example 4 according to a secondembodiment of the invention;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens of Numerical Data Example 4; and

FIG. 12 is an aberration diagram showing a spherical aberration, anastigmatism, and a distortion of the imaging lens of Numerical DataExample 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(First Embodiment)

Hereunder, referring to the accompanying drawings, a first embodiment ofthe present invention will be fully described.

FIGS. 1, 4, and 7 are sectional views of imaging lenses in NumericalData Examples 1 to 3 according to the embodiment, respectively. Since abasic lens configuration is the same among the Numerical Data Examples,the lens configuration of the embodiment will be described withreference to the lens sectional view of Numerical Data Example 1.

As shown in FIG. 1, the imaging lens of the embodiment has an aperturestop ST; a first lens L1 having positive refractive power; a second lensL2 having negative refractive power; a third lens L3 having positiverefractive power; and a fourth lens L4 having positive refractive power,arranged in this order from an object side to an image plane side. Acover glass 10 is provided between the fourth lens L4 and the imageplane. Here, the cover glass 10 may be optionally omitted. According tothe embodiment, the aperture stop is provided on a tangential plane of avertex of the object-side surface of the first lens L1. The position ofthe aperture stop is not limited to the one in this embodiment, and forexample, it may be closer to the object-side in relative to thetangential plane of the vertex of the object-side surface of the firstlens L1 or between the tangential plane of the vertex and the image-sidesurface of the first lens L1.

According to the imaging lens having the above-described configuration,the first lens L1 is formed in a shape so that a curvature radius R2 ofan object-side surface thereof is positive and a curvature radius R3 ofan image-side surface thereof is negative, i.e. a shape of a biconvexlens near the optical axis. The second lens L2 is formed in a shape sothat a curvature radius R4 of an object-side surface thereof and acurvature radius R5 of an image-side surface thereof are both positive,and has a shape of a meniscus lens directing a convex surface thereof tothe object side near the optical axis.

The third lens L3 is formed in a shape so that a curvature radius R6 ofan object-side surface thereof and a curvature radius R7 of animage-side surface thereof are both negative and has a shape of ameniscus lens directing a concave surface thereof to the object sidenear the optical axis. The fourth lens L4 is formed in a shape so that acurvature radius R8 of an object-side surface thereof and a curvatureradius R9 of an image-side surface thereof are both positive and has ashape of a meniscus lens directing a convex surface to the object sidenear the optical axis.

Furthermore, an image-side surface of the fourth lens L4is formed as anaspheric shape so that it has a convex shape on the object side near theoptical axis and a concave shape on the object side in the periphery.With the fourth lens L4having such shape, it is possible to suitablyrestrain an incident angle of a light beam emitted from the imaging lensto the image plane.

According to the imaging lens of this embodiment, a material of thefirst lens L1 and a material of the fourth lens L4 are same. Therefore,it is possible to suitably reduce the manufacturing cost in comparisonwith a case of making each of the first lens L1 to the fourth lens L4from different materials.

The imaging lens according to this embodiment satisfies the followingconditional expressions (1) to (3) and (3A). Therefore, according to theimaging lens of this embodiment, it is possible to attain bothminiaturization of the imaging lens and satisfactory aberrationcorrection.0.5<L14/f<0.8  (1)0.3<d23/L24<0.7  (2)0.1<f12/f34<0.8  (3)0.2<f12/f34<0.6  (3A)

In the above conditional expressions,

-   f: Focal length of the whole lens system-   L14: Distance on an optical axis from an object-side surface of the    first lens L1 to an image-side surface of the fourth lens L4-   d23: Distance on the optical axis from an image-side surface of the    second lens L2 to an object-side surface of the third lens L3-   L24: Distance on the optical axis from an object-side surface of the    second lens L2 to an image-side surface of the fourth lens L4-   f12: Composite focal length of the first lens L1 and the second lens    L2-   f34: Composite focal length between the third lens L3 and the fourth    lens L4

Furthermore, in order to satisfactorily correct chromatic aberrations,the imaging lens of this embodiment satisfies the conditionalexpressions (4) and (5) in addition to the conditional expressions (1)to (3) and (3A):νd1>50  (4)νd2<30  (5)

Moreover, the imaging lens of this embodiment also satisfies thefollowing conditional expressions (6) and (7):|νd1−νd4|<10  (6)|νd2−νd3|<10  (7)

When the imaging lens satisfies those conditional expressions (6) and(7), it is possible to further satisfactorily correct the axialchromatic aberration and the off-axis chromatic aberration ofmagnification.

Here, it is not necessary to satisfy all of the conditional expressions(1) to (7). When any single one of the conditional expressions isindividually satisfied, it is possible to obtain an effect correspondingto the respective conditional expression.

In the embodiment, each lens has a lens surface that is formed as anaspheric surface as necessary. When the aspheric surfaces applied to thelens surfaces have an axis Z in the optical axis direction, a height Hin a direction perpendicular to the optical axis, a conical coefficientk, and aspheric coefficients A₄, A₆, A₈, A₁₀, A₁₂, A₁₄, and A₁₆, a shapeof the aspheric surfaces of the lens surfaces may be expressed asfollows (which is the same in a second embodiment that will be describedbelow):

$\begin{matrix}{Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}} + {A_{12}H^{12}} + {A_{14}H^{14}} + {A_{16}H^{16}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Next, Numerical Data Examples of the imaging lens of the embodiment willbe described. In each of the Numerical Data Examples, f represents afocal length of a whole lens system, Fno represents an F number, and ωrepresents a half angle of view, respectively. In addition, i representsa surface number counted from the object side, R represents a curvatureradius, d represents a distance between lens surfaces (surface spacing)on the optical axis, Nd represents a refractive index for a d line, andνd represents Abbe's number for the d line, respectively. Here, asphericsurfaces are indicated with surface numbers i affixed with * (asterisk).

Numerical Data Example 1 Basic lens data are shown below. f = 4.747 mm,Fno = 2.800, ω = 31.17° Unit: mm Surface Data Surface Number i R d Nd νd(Object) ∞ ∞ 1 (Stop) ∞ 0.0000 2* 1.975 0.8300 1.52470 56.2(=νd1) 3*−3.200   0.0800 4* 234.000  0.2830 1.58500 29.0(=νd2) 5* 1.8001.2700(=d23) 6* −2.640   0.4600 1.58500 29.0(=νd3) 7* −2.700   0.0100 8*0.960 0.4500 1.52470 56.2(=νd4) 9* 0.878 0.7000 10  ∞ 0.3030 1.5163364.12 11  ∞ 0.9760 (Image ∞ plane) f12 = 6.090 f34 = 16.987 L14 = 3.383L24 = 2.473 Aspheric Surface Data Second Surface k = 1.535051,  A₄ =−3.584795E−02,  A₆ = −5.341548E−02, A₈ = 2.682726E−02, A₁₀ =−4.659743E−02 Third Surface k = −3.102858E+01,  A₄ = −7.981545E−02,  A₆= −3.753298E−02, A₈ = 7.692056E−02, A₁₀ = −5.998601E−02 Fourth Surface k= −9.972116E+39,  A₄ = −6.158896E−02,  A₆ = −1.921885E−02, A₈ =1.090468E−01, A₁₀ = −1.855461E−02, A₁₂ = −4.523068E−02, A₁₄ =5.157794E−03 Fifth Surface k = −7.153989,  A₄ = 4.468395E−02,  A₆ =1.234487E−02, A₈ = 2.658020E−02, A₁₀ = −7.028294E−02, A₁₂ =1.641493E−01, A₁₄ = −1.126544E−01 Sixth Surface k = −6.558193E+01,  A₄ =6.384066E−02,  A₆ = −5.601949E−02, A₈ = −1.598106E−03, A₁₀ =−1.218430E−03, A₁₂ = 9.262790E−05, A₁₄ = 2.751785E−03, A₁₆ =−1.141513E−03 Seventh Surface k = 6.705719E−01,  A₄ = 9.609024E−02,  A₆= −3.575929E−02, A₈ = −1.039675E−02, A₁₀ = 2.556040E−03, A₁₂ =1.189165E−03, A₁₄ = 1.056894E−04, A₁₆ = −1.310586E−04 Eighth Surface k =−3.914012,  A₄ = −8.747931E−02,  A₆ = 2.209240E−03, A₈ = 3.555385E−03,A₁₀ = −1.015133E−04, A₁₂ = −4.368905E−05, A₁₄ = −1.325858E−05, A₁₆ =2.258366E−06 Ninth Surface k = −2.979016,  A₄ = −1.045592E−01,  A₆ =2.583630E−02, A₈ = −3.520973E−03, A₁₀ = 5.731589E−06, A₁₂ =4.469465E−05, A₁₄ = −1.530316E−06, A₁₆ = −1.856164E−07

The values of the respective conditional expressions are as follows:

-   L14/f=0.713-   d23/L24=0.514-   f12/f34=0.359-   νd1=56.2-   νd2=29.0-   |νd1−νd4|=0-   |νd2−νd3|=0

Accordingly, the imaging lens of this Numerical Data Example 1 satisfiesthe respective conditional expressions (1) to (7).

In addition, the imaging lens of Numerical Data Example 1 furthersatisfies the following conditional expressions (8) and (9), so that itis possible to further satisfactorily correct the chromatic aberration:νd1=νd4  (8)νd2=νd3  (9)

In the imaging lens of Numerical Data Example 1, since a material of thesecond lens L2 and a material of the third lens L3 are same, the numberof types of materials to compose the imaging lens is only two, so thatit is possible to further reduce the manufacturing cost of the imaginglens.

FIG. 2 shows the lateral aberration that corresponds to a half angle ofview ω in the imaging lens of Numerical Data Example 1 by dividing intoa tangential direction and sagittal direction (which is also the same inFIGS. 5 and 8). Furthermore, FIG. 3 shows a spherical aberration SA(mm), an astigmatism AS (mm), and a distortion DIST (%) of the imaginglens of Numerical Data Example 1, respectively. In the aberrationdiagrams, the Offence against the Sine Condition (OSC) is also indicatedfor the spherical aberration diagram in addition to the aberrations atthe respective wavelengths of 587.56 nm, 435.84 nm, 656.27 nm, 486.13nm, and 546.07 nm. Further, in the astigmatism diagram, the aberrationon the sagittal image surface S and the aberration on tangential imagesurface T are respectively indicated (which are the same in FIGS. 6 and9). As shown in FIGS. 2 and 3, in the imaging lens of Numerical DataExample 1, each aberration is satisfactorily corrected.

Numerical Data Example 2 Basic lens data are shown below. f = 4.598 mm,Fno = 2.800, ω = 31.99° Unit: mm Surface Data Surface Number i R d Nd νd(Object) ∞ ∞ 1 (Stop) ∞ 0.0000 2* 1.960 0.8000 1.52470 56.2(=νd1) 3*−3.140   0.0800 4* 190.000  0.3000 1.61420 26.0(=νd2) 5* 1.8301.2700(=d23) 6* −2.710   0.4500 1.58500 29.0(=νd3) 7* −2.580   0.0500 8*0.937 0.4500 1.52470 56.2(=νd4) 9* 0.845 0.7000 10  ∞ 0.3000 1.5163364.12 11  ∞ 0.8828 (Image ∞ plane) f12 = 6.158 f34 = 13.611 L14 = 3.400L24 = 2.520 Aspheric Surface Data Second Surface k = 1.523272,  A₄ =−3.674821E−02,  A₆ = −5.362241E−02,  A₈ = 2.784044E−02, A₁₀ =−4.372975E−02 Third Surface k = −3.027857E+01,  A₄ = −7.891662E−02,  A₆= −3.452228E−02,  A₈ = 8.056610E−02, A₁₀ = −5.732529E−02 Fourth Surfacek = −9.972116E+39,  A₄ = −5.972871E−02,  A₆ = −1.854560E−02,  A₈ =1.097227E−01, A₁₀ = −1.673134E−02, A₁₂ = −4.210460E−02, A₁₄ =8.864766E−03 Fifth Surface k = −7.325117,  A₄ = 4.277749E−02,  A₆ =1.014590E−02,  A₈ = 2.544319E−02, A₁₀ = −6.958692E−02, A₁₂ =1.667694E−01, A₁₄ = −1.092939E−01 Sixth Surface k = −7.444598E+01,  A₄ =6.562546E−02,  A₆ = −5.667216E−02,  A₈ = −1.987852E−03, A₁₀ =−1.433377E−03, A₁₂ = −4.663016E−05, A₁₄ = 2.676482E−03, A₁₆ =−1.172270E−03 Seventh Surface k = 7.784563E−01,  A₄ = 9.590598E−02,  A₆= −3.538124E−02,  A₈ = −1.066958E−02, A₁₀ = 2.360861E−03, A₁₂ =1.114033E−03, A₁₄ = 7.983932E−05, A₁₆ = −1.405408E−04 Eighth Surface k =−3.924204,  A₄ = −8.876516E−02,  A₆ = 2.091896E−03,  A₈ = 3.546330E−03,A₁₀ = −1.020165E−04, A₁₂ = −4.365898E−05, A₁₄ = −1.324021E−05, A₁₆ =2.262549E−06 Ninth Surface k = −2.967570,  A₄ = −1.026086E−01,  A₆ =2.583143E−02,  A₈ = −3.530498E−03, A₁₀ = 4.411777E−06, A₁₂ =4.450544E−05, A₁₄ = −1.563105E−06, A₁₆ = −1.917588E−07

The values of the respective conditional expressions are as follows:

-   L14/f=0.739-   d23/L24=0.504-   f12/f34=0.452-   νd1=56.2-   νd2=26.0-   |νd1−νd4|=0-   |νd2−νd3|=3

Therefore, the imaging lens in Numerical Data Example 2 satisfies therespective conditional expressions (1) to (7).

FIG. 5 shows the lateral aberration that corresponds to a half angle ofview ω in the imaging lens of Numerical Data Example 2. Furthermore,FIG. 6 shows a spherical aberration SA (mm), an astigmatism AS (mm), anda distortion DIST (%) of the imaging lens, respectively. As shown inFIGS. 5 and 6, in the imaging lens of Numerical Data Example 2, imagesurface is satisfactorily corrected and each aberration is suitablycorrected, similarly to Numerical Data Example 1.

Numerical Data Example 3 Basic lens data are shown below. f = 4.620 mm,Fno = 2.800, ω = 31.87° Unit: mm Surface Data Surface Number i R d Nd νd(Object) ∞ ∞ 1 (Stop) ∞ 0.0000 2* 2.000 0.8000 1.52470 56.2(=νd1) 3*−2.970   0.0500 4* 85.000  0.3200 1.61420 26.0(=νd2) 5* 1.8001.2000(=d23) 6* −2.650   0.4100 1.58500 29.0(=νd3) 7* −2.580   0.0500 8*0.970 0.4500 1.52470 56.2(=νd4) 9* 0.890 0.6500 10  ∞ 0.3000 1.5163364.12 11  ∞ 1.0485 (Image ∞ plane) f12 = 6.170 f34 = 14.228 L14 = 3.280L24 = 2.430 Aspheric Surface Data Second Surface k = 1.569862,  A₄ =−3.582961E−02,  A₆ = −5.514193E−02,  A₈ = 2.111546E−02, A₁₀ =−2.100983E−02 Third Surface k = −3.448672E+01,  A₄ = −6.360458E−02,  A₆= −9.427583E−03,  A₈ = 9.524746E−02, A₁₀ = −6.873108E−02 Fourth Surfacek = 2.982380E+03,  A₄ = −1.418555E−02,  A₆ = 2.586552E−03,  A₈ =1.130570E−01, A₁₀ = −2.629102E−02, A₁₂ = −5.544998E−02, A₁₄ =1.816254E−02 Fifth Surface k = −7.256649,  A₄ = 5.014734E−02,  A₆ =2.040062E−02,  A₈ = 2.801411E−02, A₁₀ = −7.373615E−02, A₁₂ =1.620376E−01, A₁₄ = −1.064979E−01 Sixth Surface k = −5.649491E+01,  A₄ =5.512272E−02,  A₆ = −5.927976E−02,  A₈ = −3.831064E−03, A₁₀ =−1.920700E−03, A₁₂ = 2.760719E−05, A₁₄ = 2.590711E−03, A₁₆ =−1.484510E−03 Seventh Surface k = 8.337932E−01,  A₄ = 9.156358E−02,  A₆= −3.623063E−02,  A₈ = −1.090108E−02, A₁₀ = 2.123807E−03, A₁₂ =9.363177E−04, A₁₄ = 4.993016E−05, A₁₆ = −9.772932E−05 Eighth Surface k =−3.922049,  A₄ = −8.856960E−02,  A₆ = 2.891737E−03,  A₈ = 3.554763E−03,A₁₀ = −1.097780E−04, A₁₂ = −4.482765E−05, A₁₄ = −1.323961E−05, A₁₆ =2.318158E−06 Ninth Surface k = −3.004436,  A₄ = −1.045050E−01,  A₆ =2.512839E−02,  A₈ = −3.446471E−03, A₁₀ = 1.329201E−05, A₁₂ =4.482654E−05, A₁₄ = −1.526123E−06, A₁₆ = −1.704036E−07

The values of the respective conditional expressions are as follows:

-   L14/f=0.710-   d23/L24=0.494-   f12/f34=0.434-   νd1=56.2-   νd2=26.0-   |νd1−νd4|=0-   |νd2−νd3|=3

Therefore, the imaging lens in Numerical Data Example 3 satisfies therespective conditional expressions (1) to (7).

FIG. 8 shows the lateral aberration that corresponds to a half angle ofview ω in the imaging lens of Numerical Data Example 3. Furthermore,FIG. 9 shows a spherical aberration

SA (mm), an astigmatism AS (mm), and a distortion DIST (%) of theimaging lens, respectively. As shown in FIGS. 8 and 9, in the imaginglens of Numerical Data Example 3, image surface is satisfactorilycorrected and each aberration is suitably corrected, similarly toNumerical Data Example 1.

(Second Embodiment)

Next, a second embodiment of the invention will be fully describedreferring to the accompanying drawings.

FIG. 10 shows a sectional structure of an imaging lens of Numerical DataExample 4, which is a numerical data example of this embodiment. Asshown in FIG. 10, the imaging lens of this embodiment has an aperturestop ST; a first lens L1having positive refractive power; a second lensL2 having negative refractive power; a third lens L3 having negativerefractive power; and a fourth lens L4 having positive refractive powerarranged in this order from an object side to an image plane side. Acover glass 10 is provided between the fourth lens L4 and the imageplane. In addition, also in this embodiment, the aperture stop isprovided on a tangential plane of a vertex of an object-side surface ofthe first lens L1.

In the imaging lens having the above-described configuration, the firstlens L1 is formed in a shape so that a curvature radius R2 of anobject-side surface thereof is positive and a curvature radius R3 of animage-side surface thereof is negative, i.e. a shape of a biconvex lensnear the optical axis. The second lens L2 is formed in a shape so that acurvature radius R4 of an object-side surface thereof and a curvatureradius R5 of an image-side surface thereof are both positive and has ashape of a meniscus lens directing a convex surface thereof to theobject side near the optical axis.

The third lens L3 is formed in a shape so that a curvature radius R6 ofan object-side surface thereof and a curvature radius R7 of animage-side surface thereof are both negative, and has a shape of ameniscus lens directing a concave surface thereof to the object sidenear the optical axis. The fourth lens L4 is formed in a shape so that acurvature radius R8 of an object-side surface thereof and a curvatureradius R9 of an image-side surface thereof are both positive, and has ashape of a meniscus lens directing a convex surface thereof to theobject side near the optical axis.

Furthermore, the image-side surface of the fourth lens L4is formed as anaspheric shape so as to have a convex shape on the object side near theoptical axis and a concave shape on the object side in the periphery.With such shape of the fourth lens L4, it is possible to suitablyrestrain an incident angle of a light beam emitted from the imaging lensto an image plane.

Here, according to the imaging lens of this embodiment, similarly to theimaging lens of the above-described first embodiment, the material ofthe first lens L1 and the material of the fourth lens L4 are same.

The imaging lens of this embodiment satisfies the following conditionalexpressions (1) to (3) and (3A):0.5<L14/f<0.8  (1)0.3<d23/L24<0.7  (2)0.1<f12/f34<0.8  (3)0.2<f12/f34<0.6  (3A)

In the above conditional expressions,

-   f: Focal length of a whole lens system-   L14: Distance on an optical axis from an object-side surface of a    first lens L1 to an image-side surface of a fourth lens L4-   d23: Distance on the optical axis from an image-side surface of a    second lens L2 to an object-side surface of a third lens L3-   L24: Distance on the optical axis from an object-side surface of the    second lens L2 to an image-side surface of the fourth lens L4-   f12: Composite focal length of the first lens L1 and the second lens    L2-   f34: Composite focal length of the third lens L3 and the fourth lens    L4

In addition, similarly to the above-described first embodiment, theimaging lens of this embodiment satisfies the following conditionalexpressions (4) to (7) in addition to the aforementioned conditionalexpressions (1) to (3) and (3A):νd1>50  (4)νd2<30  (5)|νd1−νd4|<10  (6)|νd2−νd3|<10  (7)

Here, it is not necessary to satisfy all of the conditional expressions(1) to (7). When any single one of the conditional expressions isindividually satisfied, it is possible to obtain an effect correspondingto the respective conditional expression.

Next, Numerical Data Example of the imaging lens of the embodiment willbe described. In the Numerical Data Example, f represents a focal lengthof a whole lens system, Fno represents an F number, and ω represents ahalf angle of view, respectively. In addition, i represents a surfacenumber counted from the object side, R represents a curvature radius, drepresents a distance between lens surfaces (surface spacing) on theoptical axis, Nd represents a refractive index for a d line, and νdrepresents Abbe's number for the d line, respectively. Here, asphericsurfaces are indicated with surface numbers i affixed with * (asterisk).

Numerical Data Example 4 Basic lens data are shown below. f = 4.533 mm,Fno = 2.800, ω = 32.36° Unit: mm Surface Data Surface Number i R d Nd νd(Object) ∞ ∞ 1 (Stop) ∞ 0.0000 2* 1.750 0.6400 1.52470 56.2(=νd1) 3*−7.580   0.0500 4* 2.980 0.2750 1.61420 26.0(=νd2) 5* 1.340 1.1400(=d23)6* −1.080   0.3300 1.61420 26.0(=νd3) 7* −1.460   0.0300 8* 1.340 0.74001.52470 56.2(=νd4) 9* 1.790 0.7000 10  ∞ 0.3000 1.51633 64.12 11  ∞0.9590 (Image ∞ plane) f12 = 5.368 f34 = 16.859 L14 = 3.205 L24 = 2.515Aspheric Surface Data k = 6.786206E−01,  A₄ = −2.452177E−02,  A₆ =−4.531117E−02,  A₈ = 1.799352E−02, A₁₀ = −4.777095E−02 Third Surface k =−5.729004E+02,  A₄ = −9.783542E−02,  A₆ = 3.888487E−04,  A₈ =4.415689E−02, A₁₀ = −5.564102E−02 Fourth Surface k = 0.000000,  A₄ =−7.770589E−02,  A₆ = −1.491525E−01,  A₈ = 2.686134E−01, A₁₀ =1.252068E−01, A₁₂ = −5.138390E−01, A₁₄ = 3.257669E−01 Fifth Surface k =−3.783573,  A₄ = 6.655314E−02,  A₆ = −2.019505E−03,  A₈ = −5.264757E−02,A₁₀ = −9.946107E−02, A₁₂ = 9.168460E−01, A₁₄ = −9.366545E−01 SixthSurface k = −7.739841,  A₄ = −2.600109E−02,  A₆ = −2.781476E−02,  A₈ =1.721137E−02, A₁₀ = −6.618718E−03, A₁₂ = −1.020692E−03, A₁₄ =3.566882E−03, A₁₆ = −3.501108E−03 Seventh Surface k = −3.257123E−01,  A₄= 6.651450E−02,  A₆ = 5.551070E−03,  A₈ = 2.372390E−04, A₁₀ =1.073662E−03, A₁₂ = −1.401114E−03, A₁₄ = −6.211786E−04, A₁₆ =3.980215E−04 Eighth Surface k = −1.115632E+01,  A₄ = −1.152918E−01,  A₆= 3.977347E−02,  A₈ = −1.926234E−03, A₁₀ = −2.682682E−03, A₁₂ =4.506907E−05, A₁₄ = 1.366224E−04, A₁₆ = −7.319281E−06 Ninth Surface k =−7.835931,  A₄ = −9.770814E−02,  A₆ = 2.656391E−02,  A₈ = −3.951961E−03,A₁₀ = −3.034931E−04, A₁₂ = 3.770115E−05, A₁₄ = 3.157577E−05, A₁₆ =−6.376041E−06

The values of the respective conditional expressions are as follows:

-   L14/f=0.707-   d23/L24=0.453-   f12/f34=0.318-   νd1=56.2-   νd2=26.0-   |νd1−νd4|=0-   |νd2−νd3|=0

Therefore, the imaging lens in Numerical Data Example 4 satisfies therespective conditional expressions (1) to (7).

Furthermore, the imaging lens of this Numerical Data Example 4 furthersatisfies the following conditional expressions (8) and (9), so as tomore satisfactorily correct the chromatic aberrations.νd1=νd4  (8)νd2=νd3  (9)

Furthermore, in the imaging lens of Numerical Data Example 4, since thematerial of the second lens L2 and the material of the third lens L3 aresame, the number of types of the materials is only two, so that it ispossible to further reduce the manufacturing cost of the imaging lens.

FIG. 11 shows the lateral aberration that corresponds to a half angle ofview ω in the imaging lens of Numerical Data Example 4 by dividing intoa tangential direction and sagittal direction. Furthermore, FIG. 12shows a spherical aberration SA (mm), an astigmatism AS (mm), and adistortion DIST (%) of the imaging lens of Numerical Data Example 4,respectively. In the aberration diagrams, the Offence against the SineCondition (OSC) is also indicated for the spherical aberration diagramin addition to the aberrations at the respective wavelengths of 587.56nm, 435.84 nm, 656.27 nm, 486.13 nm, and 546.07 nm. Further, in theastigmatism diagram, the aberration on the sagittal image surface S andthe aberration on tangential image surface T are respectively indicated.As shown in FIGS. 11 and 12, in the imaging lens of Numerical DataExample 4, each aberration is satisfactorily corrected similarly to theimaging lens of the above-described first embodiment.

According to the imaging lens of this embodiment, any lens is made of aplastic lens material. Conventionally, it is usual to form the firstlens, which has high refractive power, from a glass material. A glassmaterial, however, increases the total manufacturing cost to form a lensin comparison with a case of using a plastic material, so that therestill remains a challenge in reducing the manufacturing cost of theimaging lens. According to the imaging lens of this embodiment, sinceany lens is made of a plastic material, it is possible to suitablyreduce the manufacturing cost.

Accordingly, when the imaging lens of the respective embodiments areapplied to an imaging optical system of a cellular phone, a digitalstill camera, a portable information terminal, a security camera, avehicle onboard camera, a network camera, and the like, it is possibleto attain both high performances and miniaturization of such cameras.

The invention may be applicable to the imaging lens for mounting in adevice that requires the imaging lens to attain miniaturization andsatisfactory aberration correcting performances, for example, a cellularphone or a digital still camera.

What is claimed is:
 1. An imaging lens comprising: a first lens havingpositive refractive power; a second lens having negative refractivepower; a third lens having positive refractive power; and a fourth lenshaving positive refractive power, arranged in this order from an objectside to an image plane side, wherein said first lens is formed in ashape so that a curvature radius of a surface thereof on the object sideis positive and a curvature radius of a surface thereof on the imageplane side is negative, said second lens is formed in a shape so that acurvature radius of a surface thereof on the object side and a curvatureradius of a surface thereof on the image plane side are both positive,said third lens is formed in a shape so that a curvature radius of asurface thereof on the object side and a curvature radius of a surfacethereof on the image plane side are both negative, and said first lenshas the surface on the object side away from a surface of the fourthlens on the image plane side by a distance L14 on an optical axis and anAbbe's number νd1, said second lens has an Abbe's number νd2, said thirdlens has an Abbe's number νd3, and said fourth lens has an Abbe's numberνd4 so that the following expressions are satisfied:0.5<L14/f<0.8|νd1−νd4|<10|νd2−νd3|<10 where f is a focal length of a whole lens system.
 2. Theimaging lens according to claim 1, wherein said first lens has theAbbe's number νd1, said second lens has the Abbe's number νd2, saidthird lens has the Abbe's number νd3, and said fourth lens has theAbbe's number νd4 so that the following expressions are satisfied:νd1=νd4νd2=νd3.
 3. An imaging lens comprising: a first lens having positiverefractive power; a second lens having negative refractive power; athird lens having negative refractive power; and a fourth lens havingpositive refractive power, arranged in this order from an object side toan image plane side, wherein said first lens is formed in a shape sothat a curvature radius of a surface thereof on the object side ispositive and a curvature radius of a surface thereof on the image planeside is negative, said second lens is formed in a shape so that acurvature radius of a surface thereof on the object side and a curvatureradius of a surface thereof on the image plane side are both positive,said third lens is formed in a shape so that a curvature radius of asurface thereof on the object side and a curvature radius of a surfacethereof on the image plane side are both negative, and said first lenshas the surface on the object side away from a surface of the fourthlens on the image plane side by a distance L14 on an optical axis sothat the following expression is satisfied:0.5<L14/f<0.8 where f is a focal length of a whole lens system.
 4. Theimaging lens according to claim 3, wherein said second lens has thesurface on the image plane side away from the surface of the third lenson the object side by a distance d23 on the optical axis, and saidsecond lens has the surface on the object side from the surface of thefourth lens on the image plane side by a distance L24 on the opticalaxis so that the following expression is satisfied:0.3<d23/L24<0.7.
 5. The imaging lens according to claim 3, wherein saidfirst lens and said second lens have a composite focal length f12, andsaid third lens and said fourth lens have a composite focal length f34so that the following expression is satisfied:0.1<f12/f34<0.8.
 6. The imaging lens according to claim 3, wherein saidfirst lens has an Abbe's number νd1 and said second lens has an Abbe'snumber νd2 so that the following expressions are satisfied:νd1>50νd2<30.
 7. The imaging lens according to claim 3, wherein said firstlens has the Abbe's number νd1, said second lens has the Abbe's numberνd2, said third lens has an Abbe's number νd3and said fourth lens has anAbbe's number νd4 so that the following expressions are satisfied:|νd1−νd4|<10|νd2−νd3|<10.
 8. The imaging lens according to claim 7, wherein saidfirst lens has the Abbe's number νd1, said second lens has the Abbe'snumber νd2, said third lens has the Abbe's number νd3,and said fourthlens has the Abbe's number νd4 so that the following expressions aresatisfied:νd1=νd4νd2=νd3.